The invention relates to a process for preparing an aqueous emulsion based on polyacrylates, intended for treating metal, in particular steel, surfaces so as to form, on this surface, a prelubricating dry film for protecting against corrosion.
The protection is considered to be temporary if it is not designed to be permanent; in this case, the applied film should be easy to remove or xe2x80x9ccleanxe2x80x9d when it is desired to make the metal surface xe2x80x9cbarexe2x80x9d again, for example for forming, painting or enameling.
It is desirable for the protective film to have lubricating properties, either by itself, or in synergy with oils which would be applied on the film; these oils may be whole oils or soluble oils, that is to say oil-in-water emulsions.
These lubricating properties, combined or otherwise with those of oils, are very useful for forming operations, in particular sheet metal drawing operations.
Aqueous compositions containing in suspension both oil and acrylic polymers are known.
Patent application WO 96-37SS4 by the company HENKEL describes a composition prepared from acrylic xe2x80x9cterpolymerxe2x80x9d latexes, which is intended to be applied on a metal, in particular steel, surface; these polymers are prepared from the following monomers, the values of the proportions of these monomers being given relative to the total mass of polymers in the composition:
A: methyl methacrylate or acrylonitrile, which provides good protection to the surface against corrosion (page 7, line 5) in particular if 30% less than A less than 80%.
B: ethyl or butyl acrylate, which reinforces the flexibility of the film applied on the surface (page 7, line 11) if 10% less than B less than 60%.
C: acrylic or methacrylic acid whose proportion should remain lowxe2x80x942% less than C less than 10%xe2x80x94for preserving stable emulsions and obtaining a xe2x80x9csuitablexe2x80x9d deposit on the surface.
The polymerization of the monomers is carried out in aqueous emulsion; the average molecular masses obtained, M, are between 105 and 106.
For a purely xe2x80x9ctemporaryxe2x80x9d (and nonpermanent) protection against corrosion, it is advisable to use compositions which do not risk becoming crosslinked under the application and drying conditions, so that the depositxe2x80x94or protective filmxe2x80x94obtained can be cleanable.
To improve the drawing operation for treated surfaces, that is to say to confer prelubricating properties on these compositions, it has been proposed to incorporate lubricating materials therein, as described by the documents cited below.
Patent EP 606 257 by the company PPG describes a composition intended to treat metal surfaces so as to form, on this surface, a temporary protective film against corrosion; the film obtained can be cleaned, is lubricating and is xe2x80x9cweldablexe2x80x9d.
The compositions described comprise:
(a) an acid functional acrylic polymer having as characteristics:
glass transition temperature: xe2x88x9230xc2x0 C. less than Tg less than 100xc2x0 C.,
average molecular mass: 3 000 less than M less than 100 000.
(b) a lubricating material, in a proportion of 5 to 70%, preferably 5 to 20%, xe2x80x9cin a quantity suitable for improving drawabilityxe2x80x9d chosen from a hydrocarbon wax, carnauba wax or beeswax.
(c) an aqueous-alcoholic solvent where the alcohol does not contain more than 4 carbon atoms per molecule.
The flash point of these compositions should be greater than 37.7xc2x0 C.
As an example of monomers leading to an acid functional polymer, acrylic acid, methacrylic acid and the esters of these acids are mentioned.
To incorporate the lubricating material into these compositions, it is proposed to prepare these compositions by polymerization in emulsion and in the presence of the lubricating material: xe2x80x9cin the preferred method of preparation where the polymer is prepared with the lubricating material, it is thought that there is partial grafting of the polymer with the lubricating material.xe2x80x9d (Col. 4, line 35).
Patent EP 421 250 by the company PPG describes a composition intended to treat sheet steel surfaces so as to form, on this surface, a temporary protective film against corrosion; the film obtained is cleanable, xe2x80x9cdrawablexe2x80x9d, xe2x80x9cweldablexe2x80x9d; it is also xe2x80x9cformablexe2x80x9d in the sense that the bending of a sheet metal coated with this film does not cause cracks in the film.
The compositions described comprise:
an acid function polymer prepared in solution from:
at least one monomer comprising an acid function, in a proportion xe2x89xa75% (typically 10 to 30%), such as for example an acid chosen from acrylic, methacrylic, crotonic, itaconic or maleic acid;
at least one ethylenically unsaturated monomer such as for example a styrene, a vinyltoluene, an acrylic or methacrylic acid ester such as methyl methacrylate (MMA), butyl acrylate (BuA) or 2-ethylhexyl acrylate;
an oil, in a proportion xe2x89xa75% (typically 10 to 30%), whose melting point is close to 60xc2x0 C., such as hydrocarbon wax, beeswax, carnauba wax, petroleum wax, or even a vegetable or hydrocarbon oil, or a fatty acid.
To improve the drawability of a sheet metal coated with this film and its cleanability, it is proposed to prepare these compositions by polymerization, in a solvent medium, of an organic mixture of monomers incorporating oil; the average molecular mass of the polymer obtained is between and 9xc3x97104; these compositions are then emulsified so as to be easily applied as a thin film on sheet metals.
Because of the absence of xe2x80x9cexternalxe2x80x9d surfactants, the protective film obtained is less sensitive to water, which improves the resistance to corrosion.
This document gives examples of composition (samples: A, B, C) with the proportion (%) of acrylic components (BuA: butyl acrylatexe2x80x94Styr. : styrenexe2x80x94AA: acrylic acidxe2x80x94MDAEMA:dimethylaminoethyl methacrylate) and of oil (wax) and with the glass transition temperature of the polymer obtained; Table I summarizes Examples A, B and C.
To apply the acrylic emulsion on the surface to be treated and to form a protective film, it is diluted in water until a proportion of solids of between 12% and 20% is obtained and it is applied on the sheet metal to be treated so as to obtain, after drying, a deposit of between 1 and 3 g/m2; the appropriate dilution of the solution and surface density depend on the roughness and the porosity of the substrate.
One disadvantage of the temporary protective compositions with improved lubricating properties described in documents EP 606 257 and 421 250 is that the protective film has a fatty character; this fatty character results from the proportion of oil in the composition and from the thickness of the deposit to be applied which are necessary in order to obtain both the desired effective protection and lubricating effect.
The object of the invention is to provide a process for preparing an emulsion of acrylic or methacrylic polymers for protecting against corrosion which, although containing little oil, preserves good lubricating properties while having a dry appearance after application in a film on sheet metal and drying.
The aim of the invention is also to improve the stability of the emulsions so as, in particular, to facilitate their application on sheet metal as a thin film having, in the dry state, a surface density of less than 6 g/m2.
The document JP 82 108114 A describes a process for preparing acrylic emulsions containing a wax (example of melting point: 60xc2x0 C.) intended for the hydrophobization treatment of textile fibers; according to the process described, an emulsion of acrylic monomers and an emulsion of wax are prepared separately, then the emulsions are mixed, and the mixture of emulsions is polymerized; nothing indicates that the emulsion obtained has only one homogeneous organic phase; it is even possible that the emulsion obtained is composed of two phases, one polymeric, the other oily or waxy; it therefore stems therefrom that on applying this emulsion on a sheet metal, a film would be obtained having a fatty character.
The document GB 2 007 237 relates to the use of oils grafted with a polar monomer (example: acrylic acid) as emulsifier or emulsifying agent to emulsify polymers (here: elastomers) and obtain emulsions intended to serve as adhesives for the manufacture of carpets; nothing. indicates that on using this emulsifying agent for the preparation of acrylic emulsions for protecting against corrosion, the desired prelubricating effect can be obtained on sheet metals.
The subject of the invention is a process for preparing an aqueous emulsion of polymer for the treatment of a metal surface, comprising the steps consisting in:
preparing an organic mixture comprising monomers of said polymer and a free radical polymerization initiator,
said monomers comprising at least one monomer M1 chosen from the group comprising acrylic acid esters, methacrylic acid esters and at least one acrylic or methacrylic monomer M2 having an acid, amide, amine or epoxy group,
the monomers M1 and M2 representing at least 30% of the total weight of the organic mixture and the at least one monomer M2 representing less than 25% of the total weight of the organic mixture,
in an aqueous phase containing a surfactant suitable for emulsification, miniemulsifying said organic mixture to form colloids of said mixture,
copolymerizing said monomers in a miniemulsion by activation of the initiator, characterized in that:
said organic mixture also comprises at least 0.1% by weight of oil,
the miniemulsion is prepared in the presence of at least one cosolvent added in a quantity representing at least 0.7% by weight relative to the total weight of the organic mixture and the aqueous phase, under conditions suitable for the average size of said colloids to be less than 1000 nm.
Said oil is miscible in said organic mixture in the proportions added and said cosolvent is miscible in the aqueous phase in the proportions added.
According to an advantageous characteristic of the invention, this cosolvent is based on polyalkylene glycol, which improves the tribological properties of the emulsion obtained.
Other advantageous characteristics of the process according to the invention are defined in the dependent claims.
The subject of the invention is also an aqueous emulsion for treating a metal surface, which can be obtained by the process according to the invention, comprising an aqueous phase and colloids based on polymer,
the monomeric units of said polymer comprising at least one monomer M1 chosen from the group comprising acrylic acid esters and methacrylic acid esters and at least one acrylic or methacrylic monomer M2 having an acid, amide or amine group,
the monomers M1 and M2 representing at least 30% of the total weight of said polymer and the at least one monomer M2 representing less than 25% of the total weight of said polymer,
characterized in that:
said emulsion contains at least 0.7% by weight of at least one cosolvent other than water,
said colloids contain at least 0.1% by weight of oil,
the average size of said colloids is less than 1000 nm.
Other advantageous characteristics of the treatment emulsion according to the invention are defined in the dependent claims.
The subject of the invention is also a process for protecting against corrosion of a metal surface comprising the steps consisting in:
applying to said surface an aqueous emulsion according to the invention or an emulsion prepared by the process according to the invention,
drying the applied emulsion so as to obtain a protective film,
characterized in that the surface density of said dried film is between 0.5 and 6 g/m2.
Very conclusive trials have been carried out with a density of between 1.5 and about 2 g/m2.
The subject of the invention is also a process for drawing sheet metal comprising the steps consisting in:
applying on said sheet metal an aqueous emulsion according to the invention or an emulsion prepared by the process according to the invention,
drying the applied emulsion so as to obtain a lubricating film,
actually drawing said sheet metal,
characterized in that the surface density of said dried film is between 0.5 and 5 g/m2.
Very conclusive trials have been carried out with a density of about 1 g/m2.
Preferably, for this lubricating application, said cosolvent is based on polyalkylene glycol.
Conventionally, it is advantageous to adjust the surface density of the dried film to the roughness of the sheet metal, so as to obtain the optimum prelubricating effect.
Advantageously, after the drawing operation, the sheet metals obtained are protected against corrosion by the film applied on the sheet metal.
Films having a surface density of between about 1 and about 2 g/m2 have made it possible to obtain both an effective protection against corrosion and an optimum prelubricating effect.
Advantageously, the sheet metals treated using the emulsion according to the invention, in particular for protecting against corrosion and/or for drawing, can be easily welded or cut.
The invention may also have the characteristic according to which the process comprises, before drawing, a step for oiling said sheet metal provided with said dried film, preferably produced using an aqueous emulsion of oil.
The invention will be understood more clearly on reading the description which follows, which is illustrated by trials and is given without limitation.